What is Light Pollution?
Light pollution refers to the presence of unwanted, inappropriate, or excessive artificial light. In other words, light pollution occurs when artificial lighting is incorrectly placed regardless of the time of day or night. This term is often used to describe the presence of excessive and disturbing light during sky observations or in everyday life.
Currently, approximately 83% of human habitats are affected by light pollution, and around 23% of land areas experience the phenomenon known as “skyglow,” where the night sky appears faint and diffused due to artificial lighting. Unfortunately, these rates are increasing yearly as urbanization progresses. Light pollution, a byproduct of urban development, negatively impacts human health, ecosystems, and environmental aesthetics.
In addition to skyglow, light pollution is categorized into three other types:
- Glare: Excessively bright light that causes visual discomfort.
- Light Trespass: Light that falls where it is not wanted or needed.
- Light Clutter: Excessive, confusing, and bright grouping of light sources.
What is the Bortle Scale?
The Bortle Scale, developed by John E. Bortle and published in Sky & Telescope magazine in 2001, is a scale that helps amateur astronomers compare the darkness of observation sites. The scale categorizes observation sites based on practical sky observations and light pollution levels.
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Bortle 1: Excellent Dark-Sky Site
Represents the best possible sky conditions. Zodiacal Light, Zodiacal Band, Gegenschein, and airglow can be observed effortlessly. The Triangulum Galaxy (M33) is easily visible to the naked eye without any visual aids. The Milky Way’s Scorpius and Sagittarius regions shine so brightly that they cast noticeable shadows on the ground. Naked-eye limiting magnitude can reach up to 8, and stars of magnitude 17.5 can be detected with a 32 cm telescope, and magnitude 19 with a 50 cm telescope. For an observer, Bortle 1 represents the best possible sky conditions. -
Bortle 2: Truly Dark-Sky Site
The Milky Way is highly detailed, appearing like marbled veins when viewed through a small, ordinary telescope. The Zodiacal Light is bright enough to cast faint shadows. Most globular clusters in the Messier Catalogue are visible to the naked eye. -
Bortle 3: Rural Sky
Signs of light pollution can be seen near the horizon. Clouds near the horizon are faintly illuminated, while the rest of the sky remains dark. The Milky Way, although somewhat complex, is still observable. Most of the globular clusters in the Messier Catalogue can be observed with the naked eye. The Triangulum Galaxy (M33) is easily visible using averted vision. The Zodiacal Light is still striking but appears faint. The limiting magnitude for the naked eye is around 7, and a 32 cm telescope can detect stars up to magnitude 16. -
Bortle 4: Rural/Suburban Transition
Considered a good sky by many standards, with constellations and the Milky Way clearly visible but not particularly striking. Light pollution domes from surrounding population centers are highly noticeable, illuminating clouds in the affected directions while the rest of the sky remains relatively dark. The Triangulum Galaxy (M33) can barely be seen using averted vision, and the Zodiacal Light is prominent but does not reach the zenith. Naked-eye limiting magnitude is 6.5, and a 32 cm telescope can reach up to magnitude 15.5. -
Bortle 5: Suburban Sky
The Zodiacal Light is mostly invisible, even under optimal conditions. The Milky Way is faint, even at the zenith, and nearly invisible near the horizon. Light pollution is prominent in most directions. Clouds appear brightly illuminated in most or all areas of the sky due to light pollution. The naked-eye limiting magnitude is around 6, and a 32 cm telescope can reach magnitude 15. -
Bortle 6: Bright Suburban Sky
The Zodiacal Light is entirely invisible. The Milky Way is not visible, but some faint traces might be observed near the zenith. Clouds are bright throughout the sky. The Triangulum Galaxy (M33) is not visible without a telescope, and the Andromeda Galaxy (M31) is faintly visible to the naked eye. The naked-eye limiting magnitude is 5.5, and a 32 cm telescope can detect stars up to magnitude 14.5. -
Bortle 7: Suburban/Urban Transition
The entire sky background appears vaguely grayish-white. Strong light sources and pollution are highly noticeable from all directions. The Milky Way is completely invisible. M31 is difficult to observe and barely detectable to the naked eye. Even mid-sized telescopes struggle to reveal the Messier objects in their true forms. The naked-eye limiting magnitude barely reaches 5, and a 32 cm telescope can detect stars up to magnitude 14. -
Bortle 8: City Sky
The sky appears whitish-gray or orange due to intense light pollution, making it possible to read a newspaper in the dark. M31 is only barely visible on the best nights by experienced observers. The stars forming familiar constellations are faint, with some completely invisible. Even under optimal conditions, the naked-eye limiting magnitude does not exceed 4.5, while a 32 cm telescope can reach a little over magnitude 13. -
Bortle 9: Inner-City Sky
The entire sky, even at the zenith, is brightly lit. Many stars forming familiar constellations are entirely invisible. On the best nights, only the Pleiades Cluster (M45) among the Messier objects is visible to the naked eye. Only the brightest planets, like Jupiter and Venus, and stars such as Sirius can be observed. The naked-eye limiting magnitude is 4 or less.
Light Pollution Map
You can look at a global light pollution map to find out the light pollution status of the area where you live.
